Hi! Looks like I found a bug with using Android Back button (input event), here a example how to reproduce:
Launch Unreal Engine 4.20.2 -> New Project -> Select ThirdPerson Template & create it -> Go to ThirdPersonExampleMap Blueprint -> Setup it as on screenshot below (Backspace for debug in PIE) -> Make ETC1 Android Build -> Launch it on Device and make sure all work as should (when press Android Back, game shut down)
Then
Launch the created Project -> Go to Project Setiings -> Android -> Input-> Switch Allow Bluetooth Controllers to false -> Make ETC1 Android Build -> Launch on Device -> Use a Android Back button -> Nothing happen, no input